The present invention relates to the removal of organic binders from ceramic substrate laminates. More specifically, the present invention teaches a method and sintering arrangement for achieving superior binder removal while sintering a ceramic body in an oxygen-containing environment without oxidizing any associated metallurgy.
In the field of microelectronics, ceramics serve as superior insulators and carriers for metallurgy. The ceramics can be sintered to a dense state wherein they exhibit strength as well as favorable dielectric properties. Ceramics may be pre-fired and subsequently metallized, as is done commonly for simple chip carriers; however, in packages for use in high performance computers, greater circuit densities are desirable. Greater circuit density may be realized in a multilayer body wherein unfired ceramics are cast into sheets, patterned and metallized and subsequently aligned and stacked. The stack is then laminated and fired to form a fully sintered body.
The firing of stacked and metallized ceramics involves careful selection of materials and processing conditions. The firing temperatures and environment must be compatible with the metallurgy associated with the ceramic. Furthermore, the binders and plasticizers which are added to the ceramic slurry must be removed prior to the densification of the ceramic particles. Typically, the binders and plasticizers are chosen so that they depolymerize into volatile hydrocarbons and carbonaceous residues upon heating to a particular temperature. If the carbonaceous residue (hereinafter referred to simply as carbon) is not fully removed, the ceramic obtained will be porous, weak and of inferior insulating characteristics.
The time-temperature-ambient regimen used in substrate laminate sintering is generally called the firing cycle. Various firing cycles have been proposed wherein oxygen in the form of steam is provided to the substrate laminate in carefully controlled amounts in order to facilitate the removal of the carbon. The steam reacts with the carbon to form carbon dioxide and hydrogen. As taught in Herron et al. U.S. Pat. No. 4,234,367, the partial pressure of oxygen in the ambient is controlled by introducing controlled amounts of free hydrogen with the steam, thereby recapturing any free oxygen to prohibit it from reacting with the associated metallurgy.
Herron et al. further illustrate a system wherein the associated metallurgy is copper and the ceramic is a glass ceramic. To allow any amount of the copper to be oxidized gives rise to expansion problems whereby the copper oxide formed would expand in the internal layers and place tremendous stresses on the ceramic.
Kamehara et al. U.S. Pat. No. 4,504,339 also teaches a system for firing a ceramic and metal stack in an oxygen-containing ambient. Kamehara et al. similarly claims control of the partial pressure of the oxygen at low pre-densification temperatures to prohibit oxidation of the conductors.
Herron et al. U.S. Pat. No. 4,627,160 have proposed adding a catalyst to the slurry of ceramic particles, binder and solvent as an aid in promoting the effective removal of the carbonaceous materials. Proposed for use as catalysts are copper and copper oxides. As a practical matter, the maximum amount of catalyst is limited to about 0.15 weight percent. More than this will adversely affect the strength and dielectric properties of the ceramic substrate. It has been found that the catalyst is effective in use. There, however, remains a problem in maintaining the correct ratio of hydrogen to steam during the binder burnoff phase of the firing cycle.
Brownlow et al. U.S. Pat. No. 4,474,731 proposes the use of nickel oxide as the catalyst to promote the removal of carbonaceous materials from a ceramic substrate.
Marshall U.S. Pat. No. 4,189,760 proposes a capacitor wherein layers of ceramic are coated with nickel oxide. Thereafter, the nickel oxide is reduced to nickel which may then serve as the capacitor electrodes.
Dirstine U.S. Pat. No. 4,386,985 proposes additions of nickel oxide to a ceramic capacitor to increase the dielectric constant of the capacitor. It was also found that the nickel oxide prevents the nickel electrodes from dissolving in the ceramic during sintering.
Treptow U.S. Pat. No. 2,993,815 discloses the formation of a ceramic printed circuit board wherein a layer of copper is formed on a refractory substrate. Initially, copper or copper oxide is mixed with a glass-containing paste and then applied to the surface of a green refractory substrate. Thereafter, the coated substrate is fired in an oxidizing atmosphere to remove the carbonaceous residues and also to make sure that the copper that is present is in the form of copper oxide. During this step a refractory substrate-to-glass-to-copper bond is formed. Finally, the coated substrate is heated in a reducing atmosphere to reduce the copper oxide to copper.
Kato et al. discloses a ceramic composition containing a copper oxide. During a reducing heat treatment, the copper ions move to the surface of the ceramic. Thereafter, the ceramic coated with copper ions undergoes heating in an oxidizing atmosphere followed by heating in a reducing atmosphere. The end result is a ceramic substrate having a very thin coating of copper.
The disclosure of all of the above references is incorporated by reference herein.
Notwithstanding the above teachings of those skilled in the art, there remains a real need to improve the burnoff of carbonaceous residues.
This need arises from the fact that the process to remove the carbonaceous residues as taught in, for example, the Herron et al. references is extremely slow. It would be very desirable to be able to improve the process so as to make the removal of carbonaceous residues more effective and efficient.
It is, accordingly, an object of the invention to improve the burnoff of carbonaceous residues.
It is another object of the invention to reduce the time for effective burnoff of carbonaceous residues.
It is yet another object of the invention to maintain tighter control of the ambient during burnoff of the carbonaceous residues.
These and other objects of the invention will become more apparent after referring to the following description considered in conjunction with the accompanying drawings.